1. Field of the Invention
The present invention relates to a system not only for performing real-time transmission of animation pictures, audio data, or the like, but also for transmitting/receiving digital data such as computer information, character data, or the like, in satellite broadcasting.
2. Description of the Related Art
In conventional television broadcasting, mainly transmission of video/audio signals has been made. Television broadcasting, however, has been diversified for the purposes of: delivery of information related to television programs (such as introduction of persons appearing on television programs, detailed description of commercial goods introduced on television programs, and questionnaire survey for audiences concerning the contents of television programs); and distribution of digital data such as computer programs, etc.
Digital data broadcasting at present is classified into a transmission system using ground wave (terrestrial broadcasting) and a transmission system using a satellite (satellite broadcasting). In the terrestrial broadcasting, data are multiplexed in a vertical blanking internal so that digital data are transmitted at a transmission rate of about 40 kbps. In the system in which transmission is carried out by using a satellite, digital data can be transmitted at a transmission rate of 1088 kbps.
At present, teletext broadcasting is made in the terrestrial broadcasting. The teletext broadcasting may be said to be a kind of data broadcasting because digital data are transmitted in the form of packets. In the teletext broadcasting, character data are transmitted after they are superposed in a vertical blanking interval The character data to be superposed are two-valued NRZ codes. Accordingly, the capacity of data in one horizontal scanning interval is 296 bits, so that the transmission rate is about 16 kbps per one horizontal scanning interval.
On the other hand, in the satellite broadcasting, audio signals are digitized and transmitted with an audio subcarrier of 5.727272 MHz modulated by four-differential-phase shift keying. An audio signal transmission system in the satellite broadcasting is shown in FIG. 7.
The transmission system shown in FIG. 7 has an A-mode in which audio signals are quantized linearly into a width of 14 bits by a sampling frequency of 32 kHz in an A/D portion and further compressed to a width of 10 bits, and a B-mode in which audio signals are quantized linearly into a width of 16 bits by a sampling frequency of 48 kHz.
In the A-mode, four-channel audio signals 650, 651, 652 and 653 are multiplexed by a multiplexing portion 281 and then transmitted. In the B-mode, two-channel audio signals 650 and 651 are multiplexed by the multiplexing portion 281 and then transmitted. The multiplex signal is modulated by a digital modulation portion 282, combined with a video signal 654 by a synthesizing portion 283 and transmitted to a satellite from a modulation portion 284. In the receiver side, the received signal is demodulated by a demodulation portion 285 and separated into a video signal 659 and an audio signal 660 by a separating portion 286. The audio signal 660 is demodulated by a digital demodulation portion 287 and separated into respective-channel audio signals by a separating portion 288. The respective-channel audio signals are converted into analog signals by a D/A conversion portion 289. In the A-mode, audio signals 655, 656, 657, 658 and 659 are output. In the B-mode, audio signals 655 and 656 are output. In the B-mode, high-quality audio signals can be transmitted though the number of channels is smaller than the number of channels in the A-mode.
Further, in the digital channels, digital data can be transmitted independent of the audio signals. From a frame configuration diagram, a one-frame independent digital data portion has 480 bits in the A-mode and has 224 bits in the B-mode.
Because such frames are transmitted at intervals of 1 ms, digital signals containing audio signals are transmitted at a transmission rate of 2048 kbps. The transmission of digital data is achieved when this independent digital data portion and free bits not used for transmission of audio signals are allocated to the transmission of digital data. For example, in the case of A-mode stereo without loaded audio signal, the rate of transmission of digital data is 1120 kbps (1088 kbps if management information is excluded).